Desktop app: open files in tabs instead of a single replacing side panel

## Problem

Reviewing a change across several files during one task is impractical: each newly opened file
replaces the previous one in the side panel. There is no way to keep two files open, switch
between them, or return to one after following a link, so the reader loses their place constantly.

## Desired behavior

Files open as tabs in the viewer, the way OpenAI Codex does it: several files open at once,
switching between them, and a tab stays until it is closed. A split view for comparing two files
would cover the remaining case.

## Acceptance criteria

- Opening a second file does not close the first.
- The user can switch between open files and close them individually.
- Reopening an already-open file focuses its tab instead of creating a duplicate.

## Environment

macOS (Darwin 25.6.0), Claude desktop app 1.44121.4, Claude Code 2.1.258.

No implementation files, tests, or entry points are named. Start by locating the desktop viewer's file-opening flow and its current single-panel state, then trace how links open files; done means tabs remain available, can be switched and closed, and reopening a file focuses its existing tab without duplication.

索引モデルが issue の本文から書いたものです。